1. Field of the Invention
The present invention relates to an apparatus for converting free-flowing feed material into granules, agglomerates, pellets, compacts, and the like.
2. Description of the Background Art
In the manufacture of compacts, pellets, compounds, composites, agglomerates, or granules, the feed material on which the processing is based generally has a free-flowing consistency. By means of a screw press, the feed material is brought to the processing tools, where it is appropriately processed depending on its later use.
In order to manufacture simple products, such as compacts or pellets for example, it is sufficient to compact the feed material, e.g. wood (sawdust, wood fibers, shavings, wood flour) and press it through a molding tool. The qualitative requirements on the resulting product are fairly low in this context. The manufacture of high-quality granules and agglomerates from feed material of a pure type, such as PE granules for example, entails greater construction effort. To this end, the dry feed material is delivered to an agglomerator or an extruder after being compacted.
All the aforementioned processing methods start with heavy compaction of the feed material, which causes a considerable development of heat as a result of the high pressure and intense frictional forces. During the processing of thermoplastic materials, the development of heat can be influenced in such a manner that plasticizing of the feed material occurs, followed by the formation of granules and agglomerates, which then may be used as raw materials for feeding extruders, injection molding machines, presses, and the like in later production processes. Additives such as sawdust or wood fibers can also be added to the plastics to create wood/plastic composites (WPC).
During the manufacturing process, it is critically important to observe an upper temperature limit. If the temperatures in the apparatus exceed a material-specific limit value, thermal decomposition and thus chemical conversion of the feed material occurs. Such a damaged granulate is usable only for further processing into low-quality products on account of the associated degradation of chemical and physical properties.
An apparatus suitable for this described purpose is known from DE 32 10 974 A1. This apparatus has a screw press, having an annular chamber and/or a perforated plate arranged at an end opposite the intake. During the course of the axial transport of the feed material to the annular chamber or to the perforated plate, a continuous pressure buildup takes place in the screw press. To this end, the screw press can have a pitch of the screw spirals that is variable over its length, and/or a conicity. The feed material reaches its maximum pressure immediately before and during passage through the annular chamber or perforated plate, where a considerable development of heat is present as a result of the compressive and frictional forces present. In the case of thermoplastic materials, the high temperatures cause them to soften, thus making possible their passage through the annular chamber or perforated disk.
Apparatuses are also known in which the annular chamber or perforated plate at the end of the screw press is replaced by two conical grating elements arranged coaxially one inside the other while maintaining a grating gap; the surfaces of the grating elements that face one another are equipped with grating strips. Due to a relative motion of the grating elements, the feed material forced into the grating gap by the screw press is exposed to high shear forces between the grating elements, and it agglomerates as a result of the associated frictional heat.
The disadvantage of these apparatuses resides in the fact that the feed material is exposed to high compressive and shear forces as early as in the screw, so that heating of the feed material occurs at a very early point in time. Due to the accumulation of heat energy as the feed material passes through the apparatus, the maximum temperature is often exceeded before the annular chamber, pressure plate, or grating elements are reached, with the above-described disadvantages. This problem is generally addressed by the integration of cooling devices. Nevertheless, regulation of the temperature within the apparatus is very difficult, so the temperature profile exhibits a wide range of variation, and consequently only granulate of nonuniform shape and quality are produced.
In order to solve these problems, DE 38 42 072 C1, which corresponds to, U.S. Pat. No. 5,009,586, discloses an agglomerator having a conveyor screw with material intake that guides the thermoplastic feed material axially into a disk-shaped working chamber whose outer periphery constitutes an annular compaction chamber. The outer periphery of the compaction chamber is bounded by a perforated die through which the feed material is radially pressed by a rotating pressure vane after sufficient compaction and heating. Due to the sickle-shaped design of the pressure vane, a compression zone is formed that narrows opposite the direction of rotation, into which the feed material is drawn as the pressure vane rotates and is subsequently exposed to increasing compressive forces. At the same time, the forced mixing of the feed material by the rotating pressure vane leads to high shear and frictional forces, which on the whole results in a rapid autogenous development of heat in the compaction chamber and compression zone.
The temperature rise is accompanied by a softening of the thermoplastic feed material, which in turn reduces its adhesion resistance during passage through the perforated die. Thus, when this state is achieved, no further increase in compressive and shear forces is possible, since the feed material counteracts these tendencies as a result of the associated temperature increase through a reduction in viscosity. An equilibrium state is thus established in which the temperature remains below the upper temperature limit of the feed material in question.
The advantage of this apparatus thus includes an automatic regulation of the temperature in the apparatus, so that damage to the feed material resulting from thermal overstress is precluded.
In practice, such apparatuses have proven their worth due to the constant high quality of the granules produced. However, a certain range of variation among the machine outputs of various different apparatuses is notable despite the use of identical components.